Molecular mechanisms controlling vascular lumen formation in three-dimensional extracellular matrices

Non-Excitatory Amino Acids, Melatonin, and Free Radicals: Examining the Role in Stroke and Aging

The aim of this review is to explore the relationship between melatonin, free radicals, and non-excitatory amino acids, and their role in stroke and aging. Melatonin has garnered significant attention in recent years due to its diverse physiological functions and potential therapeutic benefits by reducing oxidative stress, inflammation, and apoptosis. Melatonin has been found to mitigate ischemic brain damage caused by stroke. By scavenging free radicals and reducing oxidative damage, melatonin may help slow down the aging process and protect against age-related cognitive decline. Additionally, non-excitatory amino acids have been shown to possess neuroprotective properties, including antioxidant and anti-inflammatory in stroke and aging-related conditions. They can attenuate oxidative stress, modulate calcium homeostasis, and inhibit apoptosis, thereby safeguarding neurons against damage induced by stroke and aging processes. The intracellular accumulation of certain non-excitatory amino acids could promote harmful effects during hypoxia-ischemia episodes and thus, the blockade of the amino acid transporters involved in the process could be an alternative therapeutic strategy to reduce ischemic damage. On the other hand, the accumulation of free radicals, specifically mitochondrial reactive oxygen and nitrogen species, accelerates cellular senescence and contributes to age-related decline. Recent research suggests a complex interplay between melatonin, free radicals, and non-excitatory amino acids in stroke and aging. The neuroprotective actions of melatonin and non-excitatory amino acids converge on multiple pathways, including the regulation of calcium homeostasis, modulation of apoptosis, and reduction of inflammation. These mechanisms collectively contribute to the preservation of neuronal integrity and functions, making them promising targets for therapeutic interventions in stroke and age-related disorders.

Stabilization and improved functionality of three-dimensional perfusable microvascular networks in microfluidic devices under macromolecular crowding

BACKGROUND

There is great interest to engineer in vitro models that allow the study of complex biological processes of the microvasculature with high spatiotemporal resolution. Microfluidic systems are currently used to engineer microvasculature in vitro, which consists of perfusable microvascular networks (MVNs). These are formed through spontaneous vasculogenesis and exhibit the closest resemblance to physiological microvasculature. Unfortunately, under standard culture conditions and in the absence of co-culture with auxiliary cells as well as protease inhibitors, pure MVNs suffer from a short-lived stability.

METHODS

Herein, we introduce a strategy for stabilization of MVNs through macromolecular crowding (MMC) based on a previously established mixture of Ficoll macromolecules. The biophysical principle of MMC is based on macromolecules occupying space, thus increasing the effective concentration of other components and thereby accelerating various biological processes, such as extracellular matrix deposition. We thus hypothesized that MMC will promote the accumulation of vascular ECM (basement membrane) components and lead to a stabilization of MVN with improved functionality.

RESULTS

MMC promoted the enrichment of cellular junctions and basement membrane components, while reducing cellular contractility. The resulting advantageous balance of adhesive forces over cellular tension resulted in a significant stabilization of MVNs over time, as well as improved vascular barrier function, closely resembling that of in vivo microvasculature.

CONCLUSION

Application of MMC to MVNs in microfluidic devices provides a reliable, flexible and versatile approach to stabilize engineered microvessels under simulated physiological conditions.

Growth Factor-Free Vascularization of Marine-Origin Collagen Sponges Using Cryopreserved Stromal Vascular Fractions from Human Adipose Tissue

The successful integration of transplanted three-dimensional tissue engineering (TE) constructs depends greatly on their rapid vascularization. Therefore, it is essential to address this vascularization issue in the initial design of constructs for perfused tissues. Two of the most important variables in this regard are scaffold composition and cell sourcing. Collagens with marine origins overcome some issues associated with mammal-derived collagen while maintaining their advantages in terms of biocompatibility. Concurrently, the freshly isolated stromal vascular fraction (SVF) of adipose tissue has been proposed as an advantageous cell fraction for vascularization purposes due to its highly angiogenic properties, allowing extrinsic angiogenic growth factor-free vascularization strategies for TE applications. In this study, we aimed at understanding whether marine collagen 3D matrices could support cryopreserved human SVF in maintaining intrinsic angiogenic properties observed for fresh SVF. For this, cryopreserved human SVF was seeded on blue shark collagen sponges and cultured up to 7 days in a basal medium. The secretome profile of several angiogenesis-related factors was studied throughout culture times and correlated with the expression pattern of CD31 and CD146, which showed the formation of a prevascular network. Upon in ovo implantation, increased vessel recruitment was observed in prevascularized sponges when compared with sponges without SVF cells. Immunohistochemistry for CD31 demonstrated the improved integration of prevascularized sponges within chick chorioalantoic membrane (CAM) tissues, while in situ hybridization showed human cells lining blood vessels. These results demonstrate the potential of using cryopreserved SVF combined with marine collagen as a streamlined approach to improve the vascularization of TE constructs.

RuBPY decreases intracellular calcium by decreasing influx and increasing storage

RuBPY is a ruthenium complex NO donor that presents a nitrite in its moiety and has been shown to induce vasodilation in various arteries, as well as arterial pressure reduction with no changes in heart rate. Since vascular tone is highly dependent on the cytosolic calcium concentration ([Ca²⁺ ]c), the current study aimed to investigate the effects of RuBPY on the intracellular mobilization of calcium stores of rat aortic vascular smooth muscle cells. Vascular reactivity experiments were performed in isolated aortic rings that were contracted with a high concentration of KCl or phenylephrine (Phe). Moreover, primary cultured vascular smooth muscle cells were used to measure [Ca²⁺ ]c by confocal microscopy. The NO donor RuBPY decreased the [Ca²⁺ ]c and reduced KCl and Phe -induced contractile responses. The selective inhibitor of sarco-endoplasmic Ca-ATPase (SERCA) with thapsigargin impaired the effect of RuBPY on Phe -induced contractile response. RuBPY also reduced caffeine-induced contraction, and the contraction dependent on the capacitive Ca²⁺ influx. Therefore, our results suggest that NO released from RuBPY decreased [Ca²⁺ ]c by calcium influx blockade, and activation of guanylyl-cyclase-cGMP-GK pathway. These results indicate that RuBPY increases Ca²⁺ storage in the sarcoplasmic reticulum by SERCA activation, and also by capacitive Ca²⁺ influx inhibition, which is dependent on the intracellular release of nitric oxide from this compound. This article is protected by copyright. All rights reserved.

Zebrafish vascular quantification: a tool for quantification of three-dimensional zebrafish cerebrovascular architecture by automated image analysis

Zebrafish transgenic lines and light sheet fluorescence microscopy allow in-depth insights into three-dimensional vascular development in vivo. However, quantification of the zebrafish cerebral vasculature in 3D remains highly challenging. Here, we describe and test an image analysis workflow for 3D quantification of the total or regional zebrafish brain vasculature, called zebrafish vasculature quantification (ZVQ). It provides the first landmark- or object-based vascular inter-sample registration of the zebrafish cerebral vasculature, producing population average maps allowing rapid assessment of intra- and inter-group vascular anatomy. ZVQ also extracts a range of quantitative vascular parameters from a user-specified region of interest, including volume, surface area, density, branching points, length, radius and complexity. Application of ZVQ to 13 experimental conditions, including embryonic development, pharmacological manipulations and morpholino-induced gene knockdown, shows that ZVQ is robust, allows extraction of biologically relevant information and quantification of vascular alteration, and can provide novel insights into vascular biology. To allow dissemination, the code for quantification, a graphical user interface and workflow documentation are provided. Together, ZVQ provides the first open-source quantitative approach to assess the 3D cerebrovascular architecture in zebrafish.

Summary: An image analysis workflow pipeline for 3D quantification of the total or regional zebrafish brain vasculature, called zebrafish vasculature quantification or ZVQ.

Autophagy, not apoptosis, plays a role in lumen formation of eccrine gland organoids

Background:

Sweat secreted by eccrine sweat glands is transported to the skin surface through the lumen. The eccrine sweat gland develops from the initial solid bud to the final gland structure with a lumen, but how the lumen is formed and the mechanism of lumen formation have not yet been fully elucidated. This study aimed to investigate the mechanism of lumen formation of eccrine gland organoids (EGOs).

Methods:

Human eccrine sweat glands were isolated from the skin for tissue culture, and the primary cultured cells were collected and cultured in Matrigel for 14 days in vitro. EGOs at different development days were collected for hematoxylin and eosin (H&E) staining to observe morphological changes and for immunofluorescence staining of proliferation marker Ki67, cellular motility marker filamentous actin (F-actin), and autophagy marker LC3B. Western blotting was used to detect the expression of Ki67, F-actin, and LC3B. Moreover, apoptosis was detected using a terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) apoptosis assay kit, and the expression of poly (ADP-ribose) polymerase and Caspase-3 was detected by Western blot. In addition, 3-methyladenine (3MA) was used as an autophagy inhibitor to detect whether the formation of sweat glands can be effectively inhibited.

Results:

The results showed that a single gland cell proliferated rapidly and formed EGOs on day 4. The earliest lumen formation was observed on day 6. From day 8 to day 14, the rate of lumen formation in EGOs increased significantly. The immunofluorescence and Western blot analyses showed that the expression of Ki67 gradually decreased with the increase in days, while the F-actin expression level did not change. Notably, the expression of autophagy marker LC3B was detected in the interior cells of EGOs as the apoptosis signal of EGOs was negative. Compared with the control group, the autophagy inhibitor 3MA can effectively limit the formation rate of the lumen and reduce the inner diameter of EGOs.

Conclusion:

Using our model of eccrine gland 3D-reconstruction in Matrigel, we determined that autophagy rather than apoptosis plays a role in the lumen formation of EGOs.

Molecular basis for pericyte-induced capillary tube network assembly and maturation

Here we address the functional importance and role of pericytes in capillary tube network assembly, an essential process that is required for vascularized tissue development, maintenance, and health. Healthy capillaries may be directly capable of suppressing human disease. Considerable advances have occurred in our understanding of the molecular and signaling requirements controlling EC lumen and tube formation in 3D extracellular matrices. A combination of SCF, IL-3, SDF-1α, FGF-2 and insulin ("Factors") in conjunction with integrin- and MT1-MMP-induced signaling are required for EC sprouting behavior and tube formation under serum-free defined conditions. Pericyte recruitment to the abluminal EC tube surface results in elongated and narrow tube diameters and deposition of the vascular basement membrane. In contrast, EC tubes in the absence of pericytes continue to widen and shorten over time and fail to deposit basement membranes. Pericyte invasion, recruitment and proliferation in 3D matrices requires the presence of ECs. A detailed analysis identified that EC-derived PDGF-BB, PDGF-DD, ET-1, HB-EGF, and TGFβ1 are necessary for pericyte recruitment, proliferation, and basement membrane deposition. Blockade of these individual factors causes significant pericyte inhibition, but combined blockade profoundly interferes with these events, resulting in markedly widened EC tubes without basement membranes, like when pericytes are absent.

Effect of Microgravity on Endothelial Cell Function, Angiogenesis, and Vessel Remodeling During Wound Healing

Wound healing is a complex phenomenon that involves different cell types with various functions, i.e., keratinocytes, fibroblasts, and endothelial cells, all influenced by the action of soluble mediators and rearrangement of the extracellular matrix (ECM). Physiological angiogenesis occurs in the granulation tissue during wound healing to allow oxygen and nutrient supply and waste product removal. Angiogenesis output comes from a balance between pro- and antiangiogenic factors, which is finely regulated in a spatial and time-dependent manner, in order to avoid insufficient or excessive nonreparative neovascularization. The understanding of the factors and mechanisms that control angiogenesis and their change following unloading conditions (in a real or simulated space environment) will allow to optimize the tissue response in case of traumatic injury or medical intervention. The potential countermeasures under development to optimize the reparative angiogenesis that contributes to tissue healing on Earth will be discussed in relation to their exploitability in space.

Selective and Marked Blockade of Endothelial Sprouting Behavior Using Paclitaxel and Related Pharmacologic Agents

Whether alterations in the microtubule cytoskeleton affect the ability of endothelial cells (ECs) to sprout and form branching networks of tubes was investigated in this study. Bioassays of human EC tubulogenesis, where both sprouting behavior and lumen formation can be rigorously evaluated, were used to demonstrate that addition of the microtubule-stabilizing drugs, paclitaxel, docetaxel, ixabepilone, and epothilone B, completely interferes with EC tip cells and sprouting behavior, while allowing for EC lumen formation. In bioassays mimicking vasculogenesis using single or aggregated ECs, these drugs induce ring-like lumens from single cells or cyst-like spherical lumens from multicellular aggregates with no evidence of EC sprouting behavior. Remarkably, treatment of these cultures with a low dose of the microtubule-destabilizing drug, vinblastine, led to an identical result, with complete blockade of EC sprouting, but allowing for EC lumen formation. Administration of paclitaxel in vivo markedly interfered with angiogenic sprouting behavior in developing mouse retina, providing corroboration. These findings reveal novel biological activities for pharmacologic agents that are widely utilized in multidrug chemotherapeutic regimens for the treatment of human malignant cancers. Overall, this work demonstrates that manipulation of microtubule stability selectively interferes with the ability of ECs to sprout, a necessary step to initiate and form branched capillary tube networks.

Autocrine Hyaluronan Influences Sprouting and Lumen Formation During HUVEC Tubulogenesis In Vitro

Although many studies have focused on a role for hyaluronan (HA) of interstitial extracellular matrix (presumably produced by non-vascular "stromal" cells) in regulating vascular growth, we herein examine the influence of "autocrine HA" produced by vascular endothelial cells themselves on tubulogenesis, using human umbilical vein endothelial cells (HUVECs) in angiogenic and vasculogenic three-dimensional collagen gel cultures. Relative to unstimulated controls, tubulogenic HUVECs upregulated HAS2 mRNA and increased the synthesis of cell-associated HA (but not HA secreted into media). Confocal microscopy/immunofluorescence on cultures fixed with neutral-buffered 10% formalin (NBF) revealed cytoplasmic HAS2 in HUVEC cords and tubes. Cultures fixed with NBF (with cetylpyridinium chloride added to retain HA), stained for HA using "affinity fluorescence" (biotinylated HA-binding protein with streptavidin-fluor), and viewed by confocal microscopy showed HA throughout tube lumens, but little/no HA on the abluminal sides of the tubes or in the surrounding collagen gel. Lumen formation in angiogenic and vasculogenic cultures was strongly suppressed by metabolic inhibitors of HA synthesis (mannose and 4-methylumbelliferone). Hyaluronidase strongly inhibited lumen formation in angiogenic cultures, but not in vasculogenic cultures (where developing lumens are not open to culture medium). Collectively, our results point to a role for autocrine, luminal HA in microvascular sprouting and lumen development. (J Histochem Cytochem 69: 415-428, 2021).

PFKFB3: A Potential Key to Ocular Angiogenesis

The current treatment for ocular pathological angiogenesis mainly focuses on anti-VEGF signals. This treatment has been confirmed as effective despite the unfavorable side effects and unsatisfactory efficiency. Recently, endothelial cell metabolism, especially glycolysis, has been attracting attention as a potential treatment by an increasing number of researchers. Emerging evidence has shown that regulation of endothelial glycolysis can influence vessel sprouting. This new evidence has raised the potential for novel treatment targets that have been overlooked for a long time. In this review, we discuss the process of endothelial glycolysis as a promising target and consider regulation of the enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase as treatment for ocular pathological angiogenesis.

Salidroside improves angiogenesis-osteogenesis coupling by regulating the HIF-1α/VEGF signalling pathway in the bone environment

Angiogenesis is essential for bone formation during skeletal development. HIF-1α and the HIF-responsive gene VEGF (vascular endothelial growth factor) are reported to be a key mechanism for coupling osteogenesis and angiogenesis. Salidroside (SAL), a major biologically active compound of Rhodiola rosea L., possesses diverse pharmacological effects. However, whether SAL can protect against bone loss via the HIF-1α/VEGF pathway, specifically by inducing angiogenesis-osteogenesis coupling in vivo, remains unknown. Therefore, in the present study, we employed primary human umbilical vein endothelial cells (HUVECs) and the permanent EA.hy926 human endothelial cell line to determine the cellular and molecular effects of SAL on vascular endothelial cells and the HIF-1α-VEGF signalling pathway in the coupling of angiogenesis-osteogenesis. The in vitro study revealed that the HUVECs and EA.hy926 cells treated with conditioned medium from osteoblast cells (MG-63 cells) treated with SAL or treated directly with SAL showed enhanced proliferation, migration and capillary structure formation. However, supplementation with an anti-VEGF antibody during the treatment of endothelial cells (ECs) significantly reversed the pro-angiogenic effect of SAL. Moreover, SAL upregulated HIF-1α expression and increased its transcriptional activity, consequently upregulating VEGF expression at the mRNA and protein levels. In addition, our in vivo analysis demonstrated that SAL can stimulate endothelial sprouting from metatarsal bones. Thus, our mechanistic study demonstrated that the pro-angiogenic effects of SAL involve HIF-1α-VEGF signalling by coordinating the coupling of angiogenesis-osteogenesis in the bone environment. Therefore, we have discovered an ideal molecule that simultaneously enhances angiogenesis and osteogenesis and thereby accelerates bone healing.

Functional evaluation of prevascularization in one-stage versus two-stage tissue engineering approach of human bio-artificial muscle

A common shortcoming of current tissue engineered constructs is the lack of a functional vasculature, limiting their size and functionality. Prevascularization is a possible strategy to introduce vascular networks in these constructs. It includes among others co-culturing target cells with endothelial (precursor) cells that are able to form endothelial networks through vasculogenesis. In this paper, we compared two different prevascularization approaches of bio-artificial skeletal muscle tissue (BAM) in vitro and in vivo. In a one-stage approach, human muscle cells were directly co-cultured with endothelial cells in 3D. In a two-stage approach, a one week old BAM containing differentiated myotubes was coated with a fibrin hydrogel containing endothelial cells. The obtained endothelial networks were longer and better interconnected with the two-stage approach. We evaluated whether prevascularization had a beneficial effect on in vivo perfusion of the BAM and improved myotube survival by implantation on the fascia of the latissimus dorsi muscle of NOD/SCID mice for 5 or 14 d. Also in vivo, the two-stage approach displayed the highest vascular density. At day 14, anastomosis of implanted endothelial networks with the host vasculature was apparent. BAMs without endothelial networks contained longer and thicker myotubes in vitro, but their morphology degraded in vivo. In contrast, maintenance of myotube morphology was well supported in the two-stage prevascularized BAMs. To conclude, a two-stage prevascularization approach for muscle engineering improved the vascular density in the construct and supported myotube maintenance in vivo.

Capillary-resident EphA7+ pericytes are multipotent cells with anti-ischemic effects through capillary formation

The presence of pericytes (PCs) with multipotency and broad distribution along capillary suggests that microvasculature plays a role not only as a duct for blood fluid transport but also as a stem cell niche that contributes to tissue maintenance and regeneration. The lack of an appropriate marker for multipotent PCs still limits our understanding of their pathophysiological roles. We identified the novel marker EphA7 to detect multipotent PCs using microarray analysis of an immortalized PC library. PCs were isolated from microvessels of mouse subcutaneous adipose tissues, then EphA7⁺ PCs called capillary stem cells (CapSCs) were separated from EphA7⁻ control PCs (ctPCs) using fluorescence‐activated cell sorting system. CapSCs had highly multipotency that enabled them to differentiate into mesenchymal and neuronal lineages compared with ctPCs. CapSCs also differentiated into endothelial cells and PCs to form capillary‐like structures by themselves. Transplantation of CapSCs into ischemic tissues significantly improved blood flow recovery in hind limb ischemia mouse model due to vascular formation compared with that of ctPCs and adipose stromal cells. These data demonstrate that EphA7 identifies a subpopulation of multipotent PCs that have high angiogenesis and regenerative potency and are an attractive target for regenerative therapies.

Vascular smooth muscle contraction in hypertension

Hypertension is a major risk factor for many common chronic diseases, such as heart failure, myocardial infarction, stroke, vascular dementia, and chronic kidney disease. Pathophysiological mechanisms contributing to the development of hypertension include increased vascular resistance, determined in large part by reduced vascular diameter due to increased vascular contraction and arterial remodelling. These processes are regulated by complex-interacting systems such as the renin-angiotensin-aldosterone system, sympathetic nervous system, immune activation, and oxidative stress, which influence vascular smooth muscle function. Vascular smooth muscle cells are highly plastic and in pathological conditions undergo phenotypic changes from a contractile to a proliferative state. Vascular smooth muscle contraction is triggered by an increase in intracellular free calcium concentration ([Ca²⁺]_i), promoting actin–myosin cross-bridge formation. Growing evidence indicates that contraction is also regulated by calcium-independent mechanisms involving RhoA-Rho kinase, protein Kinase C and mitogen-activated protein kinase signalling, reactive oxygen species, and reorganization of the actin cytoskeleton. Activation of immune/inflammatory pathways and non-coding RNAs are also emerging as important regulators of vascular function. Vascular smooth muscle cell [Ca²⁺]_i not only determines the contractile state but also influences activity of many calcium-dependent transcription factors and proteins thereby impacting the cellular phenotype and function. Perturbations in vascular smooth muscle cell signalling and altered function influence vascular reactivity and tone, important determinants of vascular resistance and blood pressure. Here, we discuss mechanisms regulating vascular reactivity and contraction in physiological and pathophysiological conditions and highlight some new advances in the field, focusing specifically on hypertension.

Evaluation of Angiogenesis Assays

Angiogenesis assays allow for the evaluation of pro- or anti-angiogenic activity of endogenous or exogenous factors (stimulus or inhibitors) through investigation of their pro-or anti- proliferative, migratory, and tube formation effects on endothelial cells. To model the process of angiogenesis and the effects of biomolecules on that process, both in vitro and in vivo methods are currently used. In general, in vitro methods monitor specific stages in the angiogenesis process and are used for early evaluations, while in vivo methods more accurately simulate the living microenvironment to provide more pertinent information. We review here the current state of angiogenesis assays as well as their mechanisms, advantages, and limitations.

Human Pluripotent Stem Cells to Engineer Blood Vessels

Development of pluripotent stem cells (PSCs) is a remarkable scientific advancement that allows scientists to harness the power of regenerative medicine for potential treatment of disease using unaffected cells. PSCs provide a unique opportunity to study and combat cardiovascular diseases, which continue to claim the lives of thousands each day. Here, we discuss the differentiation of PSCs into vascular cells, investigation of the functional capabilities of the derived cells, and their utilization to engineer microvascular beds or vascular grafts for clinical application. Graphical Abstract Human iPSCs generated from patients are differentiated toward ECs and perivascular cells for use in disease modeling, microvascular bed development, or vascular graft fabrication.

Mycophenolic acid induces senescence of vascular precursor cells

OBJECTIVE

Endothelial dysfunction is central to the pathogenesis of many rheumatic diseases, typified by vascular inflammation and damage. Immunosuppressive drugs induce disease remission and lead to improved patient survival. However, there remains a higher incidence of cardiovascular disease in these patients even after adequate disease control. The purpose of this study was to determine the effect of mycophenolic acid (MPA), a commonly used immunosuppressive drug in rheumatology, on blood vessel or circulating endothelial colony forming cell number and function.

METHODS

We tested whether mycophenolic acid exerts an inhibitory effect on proliferation, clonogenic potential and vasculogenic function of endothelial colony forming cell. We also studied potential mechanisms involved in the observed effects.

RESULTS

Treatment with MPA decreased endothelial colony forming cell proliferation, clonogenic potential and vasculogenic function in a dose-dependent fashion. MPA increased senescence-associated β-galactosidase expression, p21 gene expression and p53 phosphorylation, indicative of activation of cellular senescence. Exogenous guanosine supplementation rescued diminished endothelial colony forming cell proliferation and indices of senescence, consistent with the known mechanism of action of MPA.

CONCLUSION

Our findings show that clinically relevant doses of MPA have potent anti-angiogenic and pro-senescent effects on vascular precursor cells in vitro, thus indicating that treatment with MPA can potentially affect vascular repair and regeneration. This warrants further studies in vivo to determine how MPA therapy contributes to vascular dysfunction and increased cardiovascular disease seen in patients with inflammatory rheumatic disease.

Evaluation and Characterization of Endothelial Cell Invasion and Sprouting Behavior

Here, we describe highly reproducible methods to investigate human EC invasion and sprouting behavior in 3D collagen matrices. Two assay models are presented whereby ECs are induced to sprout from a monolayer surface or from aggregated ECs suspended within a collagen gel matrix. In each case, the assays are performed using serum-free defined media containing a combination of five growth factors (Factors): FGF-2, SCF, IL-3, SDF-1α, and insulin. In both models, marked EC sprouting occurs with leading EC tip cells over a 12-24 h period. To illustrate their utility, we present data showing the influence of various pharmacologic inhibitors directed to membrane-type matrix metalloproteinases (MT-MMPs), protein kinase C alpha (PKCα), Src family kinases, and Notch-dependent signaling. Marked inhibition of sprouting is observed after blockade of MT-MMPs and PKCα, while strong increases in sprouting and EC tip cell number is observed following blockade of Src kinases, Notch signaling or both. Interestingly, the increased sprouting behavior observed following Src or Notch blockade directly correlates with a loss in the ability of ECs to form lumens. These defined in vitro assay models allow for a genetic and signaling dissection of EC tip cells vs. lumen forming ECs, which are both necessary for the formation of branching networks of tubes during vascular morphogenic events.

Subcellular Interactions during Vascular Morphogenesis in 3D Cocultures between Endothelial Cells and Fibroblasts

Background: Increasing the complexity of in vitro systems to mimic three-dimensional tissues and the cellular interactions within them will increase the reliability of data that were previously collected with in vitro systems. In vivo vascularization is based on complex and clearly defined cell–matrix and cell–cell interactions, where the extracellular matrix (ECM) seems to play a very important role. The aim of this study was to monitor and visualize the subcellular and molecular interactions between endothelial cells (ECs), fibroblasts, and their surrounding microenvironment during vascular morphogenesis in a three-dimensional coculture model. Methods: Quantitative and qualitative analyses during the generation of a coculture tissue construct consisting of endothelial cells and fibroblasts were done using transmission electron microscopy and immunohistochemistry. Results: Dynamic interactions were found in cocultures between ECs, between fibroblasts (FBs), between ECs and FBs, and between the cells and the ECM. Microvesicles were involved in intercellular information transfer. FBs took an active and physical part in the angiogenesis process. The ECM deposited by the cells triggered endothelial angiogenic activity. Capillary-like tubular structures developed and matured. Moreover, some ECM assembled into a basement membrane (BM) having three different layers equivalent to those seen in vivo. Finally, the three-dimensional in vitro construct mirrored the topography of histological tissue sections. Conclusion: Our results visualize the importance of the physical contact between all cellular and acellular components of the cocultures.

Vasculogenic and angiogenic potential of adipose stromal vascular fraction cell populations in vitro

Adipose-derived stromal vascular fraction (SVF) is a heterogeneous cell source that contains endothelial cells, pericytes, smooth muscle cells, stem cells, and other accessory immune and stromal cells. The SVF cell population has been shown to support vasculogenesis in vitro as well vascular maturation in vivo. Matrigel, an extracellular matrix (ECM) mixture has been utilized in vitro to evaluate tube formation of purified endothelial cell systems. We have developed an in vitro system that utilizes freshly isolated SVF and ECM molecules both in pure form (fibrin, laminin, collagen) as well as premixed form (Matrigel) to evaluate endothelial tip cell formation, endothelial stalk elongation, and early stages of branching and inosculation. Freshly isolated SVF rat demonstrate cell aggregation and clustering (presumptive vasculogenesis) on Matrigel ECM within the first 36 h of seeding followed by tip cell formation, stalk cell formation, branching, and inosculation (presumptive angiogenesis) during the subsequent 4 days of culture. Purified ECM molecules (laminin, fibrin, and collagen) promote cell proliferation but do not recapitulate events seen on Matrigel. We have created an in vitro system that provides a functional assay to study the mechanisms of vasculogenesis and angiogenesis in freshly isolated SVF to characterize SVF’s blood vessel forming potential prior to clinical implantation.

Junctional Adhesion Molecules (JAMs): Cell Adhesion Receptors With Pleiotropic Functions in Cell Physiology and Development

Junctional adhesion molecules (JAM)-A, -B and -C are cell-cell adhesion molecules of the immunoglobulin superfamily which are expressed by a variety of tissues, both during development and in the adult organism. Through their extracellular domains, they interact with other adhesion receptors on opposing cells. Through their cytoplasmic domains, they interact with PDZ domain-containing scaffolding and signaling proteins. In combination, these two properties regulate the assembly of signaling complexes at specific sites of cell-cell adhesion. The multitude of molecular interactions has enabled JAMs to adopt distinct cellular functions such as the regulation of cell-cell contact formation, cell migration, or mitotic spindle orientation. Not surprisingly, JAMs regulate diverse processes such as epithelial and endothelial barrier formation, hemostasis, angiogenesis, hematopoiesis, germ cell development, and the development of the central and peripheral nervous system. This review summarizes the recent progress in the understanding of JAMs, including their characteristic structural features, their molecular interactions, their cellular functions, and their contribution to a multitude of processes during vertebrate development and homeostasis.

Tumor angiogenesis and vascular normalization: alternative therapeutic targets

Tumor blood vessels are a key target for cancer therapeutic management. Tumor cells secrete high levels of pro-angiogenic factors which contribute to the creation of an abnormal vascular network characterized by disorganized, immature and permeable blood vessels, resulting in poorly perfused tumors. The hypoxic microenvironment created by impaired tumor perfusion can promote the selection of more invasive and aggressive tumor cells and can also impede the tumor-killing action of immune cells. Furthermore, abnormal tumor perfusion also reduces the diffusion of chemotherapeutic drugs and radiotherapy efficiency. To fight against this defective phenotype, the normalization of the tumor vasculature has emerged as a new therapeutic strategy. Vascular normalization, by restoring proper tumor perfusion and oxygenation, could limit tumor cell invasiveness and improve the effectiveness of anticancer treatments. In this review, we investigate the mechanisms involved in tumor angiogenesis and describe strategies used to achieve vascular normalization.

The Caenorhabditis elegans Excretory System: A Model for Tubulogenesis, Cell Fate Specification, and Plasticity

The excretory system of the nematode Caenorhabditis elegans is a superb model of tubular organogenesis involving a minimum of cells. The system consists of just three unicellular tubes (canal, duct, and pore), a secretory gland, and two associated neurons. Just as in more complex organs, cells of the excretory system must first adopt specific identities and then coordinate diverse processes to form tubes of appropriate topology, shape, connectivity, and physiological function. The unicellular topology of excretory tubes, their varied and sometimes complex shapes, and the dynamic reprogramming of cell identity and remodeling of tube connectivity that occur during larval development are particularly fascinating features of this organ. The physiological roles of the excretory system in osmoregulation and other aspects of the animal's life cycle are only beginning to be explored. The cellular mechanisms and molecular pathways used to build and shape excretory tubes appear similar to those used in both unicellular and multicellular tubes in more complex organs, such as the vertebrate vascular system and kidney, making this simple organ system a useful model for understanding disease processes.

The role of endoglin in post-ischemic revascularization

Following arterial occlusion, blood vessels respond by forming a new network of functional capillaries (angiogenesis), by reorganizing preexisting capillaries through the recruitment of smooth muscle cells to generate new arteries (arteriogenesis) and by growing and remodeling preexisting collateral arterioles into physiologically relevant arteries (collateral development). All these processes result in the recovery of organ perfusion. The importance of endoglin in post-occlusion reperfusion is sustained by several observations: (1) endoglin expression is increased in vessels showing active angiogenesis/remodeling; (2) genetic endoglin haploinsufficiency in humans causes deficient angiogenesis; and (3) the reduction of endoglin expression by gene disruption or the administration of endoglin-neutralizing antibodies reduces angiogenesis and revascularization. However, the precise role of endoglin in the several processes associated with revascularization has not been completely elucidated and, in some cases, the function ascribed to endoglin by different authors is controversial. The purpose of this review is to organize in a critical way the information available for the role of endoglin in several phenomena (angiogenesis, arteriogenesis and collateral development) associated with post-ischemic revascularization.

Auto-fusion and the shaping of neurons and tubes

Cells adopt specific shapes that are necessary for specific functions. For example, some neurons extend elaborate arborized dendrites that can contact multiple targets. Epithelial and endothelial cells can form tiny seamless unicellular tubes with an intracellular lumen. Recent advances showed that cells can auto-fuse to acquire those specific shapes. During auto-fusion, a cell merges two parts of its own plasma membrane. In contrast to cell-cell fusion or macropinocytic fission, which result in the merging or formation of two separate membrane bound compartments, auto-fusion preserves one compartment, but changes its shape. The discovery of auto-fusion in C. elegans was enabled by identification of specific protein fusogens, EFF-1 and AFF-1, that mediate cell-cell fusion. Phenotypic characterization of eff-1 and aff-1 mutants revealed that fusogen-mediated fusion of two parts of the same cell can be used to sculpt dendritic arbors, reconnect two parts of an axon after injury, or form a hollow unicellular tube. Similar auto-fusion events recently were detected in vertebrate cells, suggesting that auto-fusion could be a widely used mechanism for shaping neurons and tubes.

Human neural stem cell-induced endothelial morphogenesis requires autocrine/paracrine and juxtacrine signaling

Transplanted neural stem cells (NSC) interact with the host brain microenvironment. A neovascularization is commonly observed in the vicinity of the cell deposit, which is correlated with behavioral improvements. To elucidate the signaling mechanisms between human NSCs and endothelial cells (ECs), these were cocultured in an in vitro model in which NSC-induced endothelial morphogenesis produced a neurovascular environment. Soluble (autocrine/paracrine) and contact–mediated (juxtacrine) signaling molecules were evaluated for two conditionally immortalized fetal NSC lines derived from the cortical anlage (CTXOE03) and ganglionic eminence (STROC05), as well as an adult EC line (D3) derived from the cerebral microvasculature of a hippocampal biopsy. STROC05 were 4 times as efficient to induce endothelial morphogenesis compared to CTXOE03. The cascade of reciprocal interactions between NSCs and ECs in this process was determined by quantifying soluble factors, receptor mapping, and immunocytochemistry for extracellular matrix molecules. The mechanistic significance of these was further evaluated by pharmacological blockade. The sequential cell-specific regulation of autocrine/paracrine and juxtacrine signaling accounted for the differential efficiency of NSCs to induce endothelial morphogenesis. These in vitro studies shed new light on the reciprocal interactions between NSCs and ECs, which are pivotal for our mechanistic understanding of the efficacy of NSC transplantation.

Time to make the doughnuts: Building and shaping seamless tubes

A seamless tube is a very narrow-bore tube that is composed of a single cell with an intracellular lumen and no adherens or tight junctions along its length. Many capillaries in the vertebrate vascular system are seamless tubes. Seamless tubes also are found in invertebrate organs, including the Drosophila trachea and the Caenorhabditis elegans excretory system. Seamless tube cells can be less than a micron in diameter, and they can adopt very simple "doughnut-like" shapes or very complex, branched shapes comparable to those of neurons. The unusual topology and varied shapes of seamless tubes raise many basic cell biological questions about how cells form and maintain such structures. The prevalence of seamless tubes in the vascular system means that answering such questions has significant relevance to human health. In this review, we describe selected examples of seamless tubes in animals and discuss current models for how seamless tubes develop and are shaped, focusing particularly on insights that have come from recent studies in Drosophila and C. elegans.

Murine junctional adhesion molecules JAM-B and JAM-C mediate endothelial and stellate cell interactions during hepatic fibrosis

Classical junctional adhesion molecules JAM-A, JAM-B and JAM-C influence vascular permeability, cell polarity as well as leukocyte recruitment and immigration into inflamed tissue. As the vasculature becomes remodelled in chronically injured, fibrotic livers we aimed to determine distribution and role of junctional adhesion molecules during this pathological process. Therefore, livers of naïve or carbon tetrachloride-treated mice were analyzed by immunohistochemistry to localize all 3 classical junctional adhesion molecules. Hepatic stellate cells and endothelial cells were isolated and subjected to immunocytochemistry and flow cytometry to determine localization and functionality of JAM-B and JAM-C. Cells were further used to perform contractility and migration assays and to study endothelial tubulogenesis and pericytic coverage by hepatic stellate cells. We found that in healthy tissue, JAM-A was ubiquitously expressed whereas JAM-B and JAM-C were restricted to the vasculature. During fibrosis, JAM-B and JAM-C levels increased in endothelial cells and JAM-C was de novo generated in myofibroblastic hepatic stellate cells. Soluble JAM-C blocked contractility but increased motility in hepatic stellate cells. Furthermore, soluble JAM-C reduced endothelial tubulogenesis and endothelial cell/stellate cell interaction. Thus, during liver fibrogenesis, JAM-B and JAM-C expression increase on the vascular endothelium. More importantly, JAM-C appears on myofibroblastic hepatic stellate cells linking them as pericytes to JAM-B positive endothelial cells. This JAM-B/JAM-C mediated interaction between endothelial cells and stellate cells stabilizes vessel walls and may control the sinusoidal diameter. Increased hepatic stellate cell contraction mediated by JAM-C/JAM-C interaction may cause intrahepatic vasoconstriction, which is a major complication in liver cirrhosis.

Differential mouse-strain specific expression of Junctional Adhesion Molecule (JAM)-B in placental structures

The junctional adhesion molecule (JAM)-B, a member of the immunoglobulin superfamily, is involved in stabilization of interendothelial cell-cell contacts, formation of vascular tubes, homeostasis of stem cell niches and promotion of leukocyte adhesion and transmigration. In the human placenta, JAM-B protein is abundant and mRNA transcripts are enriched in first-trimester extravillous trophoblast in comparison to the villous trophoblast. We here aimed to elucidate the yet unexplored spatio-temporal expression of JAM-B in the mouse placenta. We investigated and semi-quantified JAM-B protein expression by immunohistochemistry in early post-implantation si tes and in mid- to late gestation placentae of various murine mating combinations. Surprisingly, the endothelium of the placental labyrinth was devoid of JAM-B expression. JAM-B was mainly present in spongiotrophoblast cells of the junctional zone, as well as in the fetal vessels of the chorionic plate, the umbilical cord and in maternal myometrial smooth muscle. We observed a strain-specific placental increase of JAM-B protein expression from mid- to late gestation in Balb/c-mated C57BL/6 females, which was absent in DBA/2J-mated Balb/c females. Due to the essential role of progesterone during gestation, we further assessed a possible modulation of JAM-B in mid-gestational placentae deficient in the progesterone receptor (Pgr(-/-)) and observed an increased expression of JAM-B in Pgr(-/-) placentae, compared to Pgr(+/+) tissue samples. We propose that JAM-B is an as yet underappreciated trophoblast lineage-specific protein, which is modulated via the progesterone receptor and shows unique strain-specific kinetics. Future work is needed to elucidate its possible contribution to placental processes necessary to ensuring its integrity, ultimately facilitating placental development and fetal growth.

Rasip1 is essential to blood vessel stability and angiogenic blood vessel growth

Cardiovascular function depends on patent, continuous and stable blood vessel formation by endothelial cells (ECs). Blood vessel development initiates by vasculogenesis, as ECs coalesce into linear aggregates and organize to form central lumens that allow blood flow. Molecular mechanisms underlying in vivo vascular 'tubulogenesis' are only beginning to be unraveled. We previously showed that the GTPase-interacting protein called Rasip1 is required for the formation of continuous vascular lumens in the early embryo. Rasip1(-/-) ECs exhibit loss of proper cell polarity and cell shape, disrupted localization of EC-EC junctions and defects in adhesion of ECs to extracellular matrix. In vitro studies showed that Rasip1 depletion in cultured ECs blocked tubulogenesis. Whether Rasip1 is required in blood vessels after their initial formation remained unclear. Here, we show that Rasip1 is essential for vessel formation and maintenance in the embryo, but not in quiescent adult vessels. Rasip1 is also required for angiogenesis in three models of blood vessel growth: in vitro matrix invasion, retinal blood vessel growth and directed in vivo angiogenesis assays. Rasip1 is thus necessary in growing embryonic blood vessels, postnatal angiogenic sprouting and remodeling, but is dispensable for maintenance of established blood vessels, making it a potential anti-angiogenic therapeutic target.

Cdc42 and k-Ras Control Endothelial Tubulogenesis through Apical Membrane and Cytoskeletal Polarization: Novel Stimulatory Roles for GTPase Effectors, the Small GTPases, Rac2 and Rap1b, and Inhibitory Influence of Arhgap31 and Rasa1

A critical and understudied property of endothelial cells is their ability to form lumens and tube networks. Although considerable information has been obtained concerning these issues, including the role of Cdc42 and Rac1 and their effectors such as Pak2, Pak4, Par6b, and co-regulators such as integrins, MT1-MMP and Par3; many key questions remain that are necessary to elucidate molecular and signaling requirements for this fundamental process. In this work, we identify new small GTPase regulators of EC tubulogenesis including k-Ras, Rac2 and Rap1b that act in conjunction with Cdc42 as well as the key downstream effectors, IQGAP1, MRCKβ, beta-Pix, GIT1, and Rasip1 (which can assemble into multiprotein complexes with key regulators including α2β1 integrin and MT1-MMP). In addition, we identify the negative regulators, Arhgap31 (by inactivating Cdc42 and Rac) and Rasa1 (by inactivating k-Ras) and the positive regulator, Arhgap29 (by inactivating RhoA) which play a major functional role during the EC tubulogenic process. Human EC siRNA suppression or mouse knockout of Rasip1 leads to identical phenotypes where ECs form extensive cord networks, but cannot generate lumens or tubes. Essential roles for these molecules during EC tubulogenesis include; i) establishment of asymmetric EC cytoskeletal polarization (subapical distribution of acetylated tubulin and basal membrane distribution of F-actin); and ii) directed membrane trafficking of pinocytic vacuoles or other intracellular vesicles along acetylated tubulin tracks to the developing apical membrane surface. Cdc42 co-localizes subapically with acetylated tubulin, while Rac1 and k-Ras strongly label vacuole/ vesicle membranes which accumulate and fuse together in a polarized, perinuclear manner. We observe polarized apical membrane and subapical accumulation of key GTPases and effectors regulating EC lumen formation including Cdc42, Rac1, Rac2, k-Ras, Rap1b, activated c-Raf and Rasip1 to control EC tube network assembly. Overall, this work defines novel key regulators and their functional roles during human EC tubulogenesis.

Walking the Line: A Fibronectin Fiber-Guided Assay to Probe Early Steps of (Lymph)angiogenesis

Angiogenesis and lymphangiogenesis are highly complex morphogenetic processes, central to many physiological and pathological conditions, including development, cancer metastasis, inflammation and wound healing. While it is described that extracellular matrix (ECM) fibers are involved in the spatiotemporal regulation of angiogenesis, current angiogenesis assays are not specifically designed to dissect and quantify the underlying molecular mechanisms of how the fibrillar nature of ECM regulates vessel sprouting. Even less is known about the role of the fibrillar ECM during the early stages of lymphangiogenesis. To address such questions, we introduced here an in vitro (lymph)angiogenesis assay, where we used microbeads coated with endothelial cells as simple sprouting sources and deposited them on single Fn fibers used as substrates to mimic fibrillar ECM. The fibers were deposited on a transparent substrate, suitable for live microscopic observation of the ensuing cell outgrowth events at the single cell level. Our proof-of-concept studies revealed that fibrillar Fn, compared to Fn-coated surfaces, provides far stronger sprouting and guidance cues to endothelial cells, independent of the tested mechanical strains of the Fn fibers. Additionally, we found that VEGF-A, but not VEGF-C, stimulates the collective outgrowth of lymphatic endothelial cells (LEC), while the collective outgrowth of blood vascular endothelial cells (HUVEC) was prominent even in the absence of these angiogenic factors. In addition to the findings presented here, the modularity of our assay allows for the use of different ECM or synthetic fibers as substrates, as well as of other cell types, thus expanding the range of applications in vascular biology and beyond.

Matrix metalloproteinase 14 modulates signal transduction and angiogenesis in the cornea

The cornea is transparent and avascular, and retention of these characteristics is critical to maintaining vision clarity. Under normal conditions, wound healing in response to corneal injury occurs without the formation of new blood vessels; however, neovascularization may be induced during corneal wound healing when the balance between proangiogenic and antiangiogenic mediators is disrupted to favor angiogenesis. Matrix metalloproteinases (MMPs), which are key factors in extracellular matrix remodeling and angiogenesis, contribute to the maintenance of this balance, and in pathologic instances, can contribute to its disruption. Here, we elaborate on the facilitative role of MMPs, specifically MMP-14, in corneal neovascularization. MMP-14 is a transmembrane MMP that is critically involved in extracellular matrix proteolysis, exosome transport, and cellular migration and invasion, processes that are critical for angiogenesis. To aid in developing efficacious therapies that promote healing without neovascularization, it is important to understand and further investigate the complex pathways related to MMP-14 signaling, which can also involve vascular endothelial growth factor, basic fibroblast growth factor, Wnt/β-catenin, transforming growth factor, platelet-derived growth factor, hepatocyte growth factor or chemokines, epidermal growth factor, prostaglandin E2, thrombin, integrins, Notch, Toll-like receptors, PI3k/Akt, Src, RhoA/RhoA kinase, and extracellular signal-related kinase. The involvement and potential contribution of these signaling molecules or proteins in neovascularization are the focus of the present review.

Investigating human vascular tube morphogenesis and maturation using endothelial cell-pericyte co-cultures and a doxycycline-inducible genetic system in 3D extracellular matrices

Considerable progress has occurred toward our understanding of the molecular basis for vascular morphogenesis, maturation, and stabilization. A major reason for this progress has been the development of novel in vitro systems to investigate these processes in 3D extracellular matrices. In this chapter, we present models of human endothelial cell (EC) tube formation and EC-pericyte tube co-assembly using serum-free defined conditions in 3D collagen matrices. We utilize both human venous and arterial ECs and show that both cell types readily form tubes and induce pericyte recruitment and both ECs and pericytes work together to remodel the extracellular matrix environment by assembling the vascular basement membrane, a key step in capillary tube network maturation and stabilization. Importantly, we have shown that these events occur under serum-free defined conditions using the hematopoietic stem cell cytokines, SCF, IL-3, and SDF-1α and also including FGF-2. In contrast, the combination of VEGF and FGF-2 fails to support vascular tube morphogenesis or pericyte-induced tube maturation under the same serum-free defined conditions. Furthermore, we present novel assays whereby we have developed both human ECs and pericytes to induce specific genes using a doxycycline-regulated lentiviral system. In this manner, we can upregulate the expression of wild-type or mutant gene products at any stage of vascular morphogenesis or maturation in 3D matrices. These in vitro experimental approaches will continue to identify key molecular requirements and signaling pathways that control fundamental events in tissue vascularization under normal or pathologic conditions. Furthermore, these models will provide new insights into the development of novel disease therapeutic approaches where vascularization is an important pathogenic component and create new ways to assemble capillary tube networks with associated pericytes for tissue engineering applications.

Phosphorylation of Threonine 794 on Tie1 by Rac1/PAK1 Reveals a Novel Angiogenesis Regulatory Pathway

The endothelial receptor tyrosine kinase (RTK) Tie1 was discovered over 20 years ago, yet its precise function and mode of action remain enigmatic. To shed light on Tie1’s role in endothelial cell biology, we investigated a potential threonine phosphorylation site within the juxtamembrane domain of Tie1. Expression of a non-phosphorylatable mutant of this site (T794A) in zebrafish (Danio rerio) significantly disrupted vascular development, resulting in fish with stunted and poorly branched intersomitic vessels. Similarly, T794A-expressing human umbilical vein endothelial cells formed significantly shorter tubes with fewer branches in three-dimensional Matrigel cultures. However, mutation of T794 did not alter Tie1 or Tie2 tyrosine phosphorylation or downstream signaling in any detectable way, suggesting that T794 phosphorylation may regulate a Tie1 function independent of its RTK properties. Although T794 is within a consensus Akt phosphorylation site, we were unable to identify a physiological activator of Akt that could induce T794 phosphorylation, suggesting that Akt is not the physiological Tie1-T794 kinase. However, the small GTPase Ras-related C3 botulinum toxin substrate 1 (Rac1), which is required for angiogenesis and capillary morphogenesis, was found to associate with phospho-T794 but not the non-phosphorylatable T794A mutant. Pharmacological activation of Rac1 induced downstream activation of p21-activated kinase (PAK1) and T794 phosphorylation in vitro, and inhibition of PAK1 abrogated T794 phosphorylation. Our results provide the first demonstration of a signaling pathway mediated by Tie1 in endothelial cells, and they suggest that a novel feedback loop involving Rac1/PAK1 mediated phosphorylation of Tie1 on T794 is required for proper angiogenesis.

Single-cell analysis of endothelial morphogenesis in vivo

Vessel formation has been extensively studied at the tissue level, but the difficulty in imaging the endothelium with cellular resolution has hampered study of the morphogenesis and behavior of endothelial cells (ECs) in vivo. We are using endothelial-specific transgenes and high-resolution imaging to examine single ECs in zebrafish. By generating mosaics with transgenes that simultaneously mark endothelial nuclei and membranes we are able to definitively identify and study the morphology and behavior of individual ECs during vessel sprouting and lumen formation. Using these methods, we show that developing trunk vessels are composed of ECs of varying morphology, and that single-cell analysis can be used to quantitate alterations in morphology and dynamics in ECs that are defective in proper guidance and patterning. Finally, we use single-cell analysis of intersegmental vessels undergoing lumen formation to demonstrate the coexistence of seamless transcellular lumens and single or multicellular enclosed lumens with autocellular or intercellular junctions, suggesting that heterogeneous mechanisms contribute to vascular lumen formation in vivo. The tools that we have developed for single EC analysis should facilitate further rigorous qualitative and quantitative analysis of EC morphology and behavior in vivo.

Cdc42 is required for cytoskeletal support of endothelial cell adhesion during blood vessel formation in mice

The Rho family of small GTPases has been shown to be required in endothelial cells (ECs) during blood vessel formation. However, the underlying cellular events controlled by different GTPases remain unclear. Here, we assess the cellular mechanisms by which Cdc42 regulates mammalian vascular morphogenesis and maintenance. In vivo deletion of Cdc42 in embryonic ECs (Cdc42(Tie2KO)) results in blocked lumen formation and endothelial tearing, leading to lethality of mutant embryos by E9-10 due to failed blood circulation. Similarly, inducible deletion of Cdc42 (Cdc42(Cad5KO)) at mid-gestation blocks angiogenic tubulogenesis. By contrast, deletion of Cdc42 in postnatal retinal vessels leads to aberrant vascular remodeling and sprouting, as well as markedly reduced filopodia formation. We find that Cdc42 is essential for organization of EC adhesion, as its loss results in disorganized cell-cell junctions and reduced focal adhesions. Endothelial polarity is also rapidly lost upon Cdc42 deletion, as seen by failed localization of apical podocalyxin (PODXL) and basal actin. We link observed failures to a defect in F-actin organization, both in vitro and in vivo, which secondarily impairs EC adhesion and polarity. We also identify Cdc42 effectors Pak2/4 and N-WASP, as well as the actomyosin machinery, to be crucial for EC actin organization. This work supports the notion of Cdc42 as a central regulator of the cellular machinery in ECs that drives blood vessel formation.

Human platelet lysate improves human cord blood derived ECFC survival and vasculogenesis in three dimensional (3D) collagen matrices

Human cord blood (CB) is enriched in circulating endothelial colony forming cells (ECFCs) that display high proliferative potential and in vivo vessel forming ability. Since diminished ECFC survival is known to dampen the vasculogenic response in vivo, we tested how long implanted ECFC survive and generate vessels in three-dimensional (3D) type I collagen matrices in vitro and in vivo. We hypothesized that human platelet lysate (HPL) would promote cell survival and enhance vasculogenesis in the 3D collagen matrices. We report that the percentage of ECFC co-cultured with HPL that were alive was significantly enhanced on days 1 and 3 post-matrix formation, compared to ECFC alone containing matrices. Also, co-culture of ECFC with HPL displayed significantly more vasculogenic activity compared to ECFC alone and expressed significantly more pro-survival molecules (pAkt, p-Bad and Bcl-xL) in the 3D collagen matrices in vitro. Treatment with Akt1 inhibitor (A-674563), Akt2 inhibitor (CCT128930) and Bcl-xL inhibitor (ABT-263/Navitoclax) significantly decreased the cell survival and vasculogenesis of ECFC co-cultured with or without HPL and implicated activation of the Akt1 pathway as the critical mediator of the HPL effect on ECFC in vitro. A significantly greater average vessel number and total vascular area of human CD31(+) vessels were present in implants containing ECFC and HPL, compared to the ECFC alone implants in vivo. We conclude that implantation of ECFC with HPL in vivo promotes vasculogenesis and augments blood vessel formation via diminishing apoptosis of the implanted ECFC.

Defective pericyte recruitment of villous stromal vessels as the possible etiologic cause of hydropic change in complete hydatidiform mole

The pathogenetic mechanism underlying the hydropic change in complete hydatidiform moles (CHMs) is poorly understood. A growing body of data suggests that pericytes play a role in vascular maturation. Since maturation of villous stromal vessels in CHMs is markedly impaired at early stages, we postulated that a defect in pericytes around stromal vessels in chorionic villi might cause vascular immaturity and subsequent hydropic change. To investigate this, we examined several markers of pericytes, namely, α-smooth muscle actin (α-SMA), platelet-derived growth factor receptor-β (PDGFR-β), and desmin, in 61 normally developing placentas and 41 CHMs with gestational ages of 4-12 weeks. The ultrastructure of villous stromal vessels was also examined. Mature blood vessels from normal placentas show patent vascular lumens and formed hematopoietic components in the villous stroma. α-SMA and PDGFR-β expression in the villous stroma gradually increased and extended from the chorionic plate to peripheral villous branches. The labeled cells formed a reticular network in the villous stroma and, after week 7, encircled villous stromal vessels. In comparison, α-SMA and PDGFR-β expression in the villous stroma and stromal vessels of CHMs was significantly lower (p<0.05). Ultrastructurally, endothelial cells in villous stromal vessels in normal placentas were consistently attached by pericytes after week 7 when the vessels formed distinct lumen, whereas the villous stromal vessels in CHMs consisted of linear chains of endothelial cells, often disclosing primitive clefts without hematopoietic cells inside, and neither pericytes nor basal lamina surrounded the endothelial cells at any gestational age studied. This suggests that pericytes recruitment around villous stromal vessels is defective in CHMs and links to the persistent vascular immaturity of the villous stroma in CHMs, which in turns leads to hydropic villi.

Outside in: inversion of cell polarity controls epithelial lumen formation

Establishment of cell polarity is important for epithelial lumen formation, and the molecular mechanisms directing this process are only partially understood. In this issue of Developmental Cell, Bryant et al. (2014) show that disassembly, membrane translocation, and reassembly of podocalyxin complexes controls epithelial cell polarization and lumen formation in 3D matrices.

Molecular control of capillary morphogenesis and maturation by recognition and remodeling of the extracellular matrix: functional roles of endothelial cells and pericytes in health and disease

This review addresses fundamental mechanisms underlying how capillaries form in three-dimensional extracellular matrices and how endothelial cells (ECs) and pericytes co-assemble to form capillary networks. In addition to playing a critical role in supplying oxygen and nutrients to tissues, recent work suggests that blood vessels supply important signals to facilitate tissue development. Here, we hypothesize that another major function of capillaries is to supply signals to suppress major disease mechanisms including inflammation, infection, thrombosis, hemorrhage, edema, ischemic injury, fibrosis, autoimmune disease and tumor growth/progression. Capillary dysfunction plays a key pathogenic role in many human diseases, and thus, this suppressing function may be attenuated and central toward the initiation and progression of disease. We describe how capillaries form through creation of EC-lined tube networks and vascular guidance tunnels in 3D extracellular matrices. Pericytes recruit to the abluminal EC tube surface within these tunnel spaces, and work together to assemble the vascular basement membrane matrix. These processes occur under serum-free conditions in 3D collagen or fibrin matrices and in response to five key growth factors which are stem cell factor, interleukin-3, stromal-derived factor-1α, fibroblast growth factor-2 and insulin. In addition, we identified a key role for EC-derived platelet-derived growth factor-BB and heparin-binding epidermal growth factor in pericyte recruitment and proliferation to promote EC-pericyte tube co-assembly and vascular basement membrane matrix deposition. A molecular understanding of capillary morphogenesis and maturation should lead to novel therapeutic strategies to repair capillary dysfunction in major human disease contexts including cancer and diabetes.

Adipose tissue-derived stromal cells acquire endothelial-like features upon reprogramming with SOX18

Adipose tissue-derived stromal cells (ASC) form a rich source of autologous cells for use in regenerative medicine. In vitro induction of an endothelial phenotype may improve performance of ASCs in cardiovascular repair. Here, we report on an in vitro strategy using direct reprogramming of ASCs by means of ectopic expression of the endothelial-specific transcription factor SRY (sex determining region Y)-box18 (SOX18). SOX18 induces ASCs to express a set of genes involved in vascular patterning: MMP7, KDR, EFNB2, SEMA3G and CXCR4. Accordingly, SOX18 transduced ASCs reorganize under conditions of shear stress, display VEGF-induced chemotaxis and form tubular structures in 3D matrices in an MMP7-dependent manner. These in vitro findings provide insight into molecular and cellular processes downstream of SOX18 and show that reprogramming using SOX18 is sufficient to induce several endothelial-like features in ASCs.

Inhibition of tumor-associated αvβ3 integrin regulates the angiogenic switch by enhancing expression of IGFBP-4 leading to reduced melanoma growth and angiogenesis in vivo

A more complete understanding of the mechanisms that regulate the angiogenic switch, which contributes to the conversion of small dormant tumors to actively growing malignancies, is important for the development of more effective anti-angiogenic strategies for cancer therapy. While significant progress has been made in understanding the complex mechanisms by which integrin αvβ3 expressed in endothelial cells governs angiogenesis, less is known concerning the ability of αvβ3 expressed within the tumor cell compartment to modulate the angiogenic output of a tumor. Here we provide evidence that αvβ3 expressed in melanoma cells may contribute to the suppression of IGFBP-4, an important negative regulator of IGF-1 signaling. Given the multiple context-dependent roles for αvβ3 in angiogenesis and tumor progression, our novel findings provide additional molecular insight into how αvβ3 may govern the angiogenic switch by a mechanism associated with a p38 MAPK and matrix metalloproteinases-dependent regulation of the endogenous angiogenesis inhibitor IGFBP-4.

Fabrication and in vivo microanastomosis of vascularized tissue-engineered constructs

Tissue engineering endeavors to create replacement tissues and restore function that may be lost through infection, trauma, and cancer. However, wide clinical application of engineered scaffolds has yet to come to fruition due to inadequate vascularization. Here, we fabricate hydrogel constructs using Pluronic(®) F127 as a sacrificial microfiber, creating microchannels within biocompatible, biodegradable type I collagen matrices. Microchannels were seeded with human umbilical vein endothelial cells (HUVEC) or HUVEC and human aortic smooth muscle cells (HASMC) in co-culture, generating constructs with an internal endothelialized microchannel. Histological analysis demonstrated HASMC/HUVEC-seeded constructs with a confluent lining after 7 days with preservation and further maturation of the lining after 14 days. Immunohistochemical staining demonstrated von Willebrand factor and CD31(+) endothelial cells along the luminal surface (neointima) and alpha-smooth muscle actin expressing smooth muscle cells in the subendothelial plane (neomedia). Additionally, the deposition of extracellular matrix (ECM) components, heparan sulfate and basal lamina collagen IV were detected after 14 days of culture. HUVEC-only- and HASMC/HUVEC-seeded microchannel-containing constructs were microsurgically anastomosed to rat femoral artery and vein and perfused, in vivo. Both HUVEC only and HUVEC/HAMSC-seeded constructs withstood physiologic perfusion pressures while their channels maintained their internal infrastructure. In conclusion, we have synthesized and performed microvascular anastomosis of tissue-engineered hydrogel constructs. This represents a significant advancement toward the generation of vascularized tissues and brings us closer to the fabrication of more complex tissues and solid organs for clinical application.

Interferon-induced transmembrane protein 1 regulates endothelial lumen formation during angiogenesis

OBJECTIVE

It is well established that angiogenesis is a complex and coordinated multistep process. However, there remains a lack of information about the genes that regulate individual stages of vessel formation. Here, we aimed to define the role of human interferon-induced transmembrane protein 1 (IFITM1) during blood vessel formation.

APPROACH AND RESULTS

We identified IFITM1 in a microarray screen for genes differentially regulated by endothelial cells (ECs) during an in vitro angiogenesis assay and found that IFITM1 expression was strongly induced as ECs sprouted and formed lumens. We showed by immunohistochemistry that human IFITM1 was expressed by stable blood vessels in multiple organs. siRNA-mediated knockdown of IFITM1 expression spared EC sprouting but completely disrupted lumen formation, in both in vitro and in an in vivo xeno-transplant model. ECs lacking IFITM1 underwent early stages of lumenogenesis (ie, intracellular vacuole formation) but failed to mature or expand lumens. Coimmunoprecipitation studies confirmed occludin as an IFITM1 binding partner in ECs, and immunocytochemistry showed a lack of occludin at endothelial tight junctions in the absence of IFITM1. Finally, time-lapse video microscopy revealed that IFITM1 is required for the formation of stable cell-cell contacts during endothelial lumen formation.

CONCLUSIONS

IFITM1 is essential for the formation of functional blood vessels and stabilizes EC-EC interactions during endothelial lumen formation by regulating tight junction assembly.